1. Field of the Invention
The present invention relates to a method of laminating polyolefin-based resin films or sheets by high-frequency (HF) dielectric heating, and to HF-sealed articles of the films or sheets having specific physical properties at the sealed site.
2. Description of the Related Art
In the field of wrapping or packaging materials, stationery, toys, daily necessities and others, generally used are synthetic resin articles as produced from various synthetic resin films and sheets through secondary working of, for example, cutting, laminating and sealing them.
Various methods are employed for secondary working of laminating such shaped synthetic resin materials. The most simple method is of heat-sealing the materials by the use of hot plates and the like. However, this takes a lot of time for sealing and therefore its productivity is low. In addition, since it is difficult to uniformly heat the entire region to be sealed in the method, the outward appearance of the sealed site is often not good. Another method is known of applying ultrasonic vibration to the materials being sealed. However, its effect is not satisfactory in laminating flexible materials such as polyolefin resin materials. Still another method is known of utilizing high-frequency dielectric heating. Its advantages are that molds with a simple structure can be used, with which sealing and cutting can be effected at the same time, and that sealed articles with any desired design can be produced. However, the method is not suitable to polyolefin resin materials having a small dielectric constant and a small dielectric loss coefficient, and is therefore employed exclusively for sealing of polyvinyl chloride resin materials.
Because of the problem of recent environmental pollution with them, polyvinyl chloride resin materials are being kept away from commercial applications. In that situation, the synthetic resin articles in the field in which polyvinyl chloride resin articles have heretofore been used are being gradually replaced with polyolefin resin articles. With that tendency, advantageous sealing by high-frequency dielectric heating is being tried for the production of sealed polyolefin resin articles.
For example, JP-A-55-61435 has proposed a method of high-frequency dielectric heating sealing of polyolefin resin materials, in which a metal conductor element of iron or the like is positioned in places on the resin surfaces to be sealed so that the heat of the metal conductor element as heated by high-frequency voltage applied thereto is transferred to the resin materials. In this method, the metal conductor element of iron powder or the like is contacted with the polyolefin resin materials. Therefore, the method is problematic in that the color and the outward appearance of the sealed articles will be worsened.
JP-A-51-119771 has proposed a high-frequency dielectric heating method for laminating polyolefin resin materials, in which a polar resin sheet is contacted with the polyolefin resin material. JP-A-1-160633 has proposed a technique of contacting a sheet of any of chlorosulfonated polyethylene, chlorinated polyethylene or ethylene-vinyl acetate copolymer with polyolefin resin materials being sealed by high-frequency dielectric heating. JP-A-7-5711 has proposed sealing of copolymer sheets having carbonyl groups as introduced into the polyethylene chain. JP-A-8-52802 has proposed a high-frequency dielectric heating method for sealing, in which the mold and the level block to be used are previously heated at a temperature somewhat lower than the melting point of the materials to be sealed, prior to laminating the materials by high-frequency dielectric heating.
In those methods, polyolefin resin materials could be sealed by high-frequency dielectric heating. However, the sealed articles as obtained according to those methods are problematic in that the mechanical strength of the sealed site of the articles is not satisfactory and that, when materials having high transparency are sealed, the transparency of the sealed site of the sealed articles is lowered. In addition, sparking often occurs in the process of such high-frequency dielectric heating sealing. For these reasons, it is desired to develop a sealing method capable of realizing stable production of sealed articles with no such problems.